On the modeling of calcium sulfate solubility in aqueous solutions

The electrolyte-NRTL model used in a previous work (B. Messnaoui. T. Bounahmidi. Fluid Phase Equilibr. 237(1-2) (2005) 77-85), was extended for modeling of alpha-hemihydrate and gypsum solubilities in the complex system Ca2+-H+-SO42--HSO4--H2PO4--H3PO4-H2O, at wide range of temperature and P2O5 concentration. The chemical equilibrium constant was evaluated as function of temperature according to the Gibbs-Helmholtz equation. The temperature dependence was taken into account in the expression of the standard state heat capacity of ionic, molecular and cristalline salts species. The standard-state heat capacity of Ca2+ at 298.15 K is calculated to be 27.30J mol(-1) K-1. It is also shown that the experimental data agree with the predicted values of gypsum and anhydrite solubilities in water. at high temperature. by using only the values of parameters tau(2+)(2-)(H2O-(Ca)(.SO4)), tau(2+)(2-)((Ca)(.SO4))-(H2O) which were calculated, at 298.15 K from data of gypsum solubilities in phosphoric acid solutions. (c) 2006 Elsevier B.V. All rights reserved.